1. Field of the Invention
The present invention relates to a cylinder for stopping a piston accurately in a desired position with a small-size motor by displacing the piston with a fluid under pressure and the motor, and more particularly to a servo cylinder apparatus in which a valve is integrally mounted on such a cylinder for accurately controlling the piston.
2. Description of the Related Art
There have heretofore been known actuators for displacing a workpiece on a table that is fixed to the distal end of the piston rod of a cylinder. One actuator structure has a piston rod coupled to a piston which is housed in a cylinder chamber for axial displacement therein under a pneumatic pressure. According to another actuator design, a piston rod is connected to a ball screw which is rotated by an electric motor or the like for displacing the piston rod.
The former actuator structure is capable of displacing a heavy workpiece as it can produces large output power. However, since the air is a compressible fluid, it is difficult to position the workpiece accurately.
The latter actuator design can stop a workpiece in an accurate position as the electric motor is electrically controlled. However, if a heavy workpiece is to be conveyed and positioned, then it is necessary to employ an electric motor capable of generating large output power, and such an electric motor is necessarily large in size.
In view of the above conventional actuator drawbacks, there has been proposed a hybrid actuator system which has a cylinder for conveying a workpiece under pneumatic pressure to a zone near a position in which the workpiece is to be stopped, and an electric motor for moving the workpiece from the zone and stopping it accurately in the position. Since the electric motor which is used may be small in size, the proposed hybrid actuator system may also be small in size. The proposed hybrid actuator system is also advantageous in that it can stop the workpiece accurately in a desired position.
The proposed hybrid actuator system has a solenoid-operated directional control valve disposed in fluid passages for supplying and discharging the pneumatic pressure to and from the cylinder. If a brake mechanism for stopping the workpiece is operated also under pneumatic pressure, then the brake mechanism is associated with a solenoid-operated directional control valve. As the distance between the cylinder and its solenoid-operated directional control valve or the distance between the brake mechanism and its solenoid-operated directional control valve increases, the time required for a pneumatic pressure action from the solenoid-operated directional control valve to reach the cylinder or the brake mechanism also increases. The increased distance results in an excessively long response time of the cylinder or the brake mechanism. If the brake mechanism has a long response time, then it takes a relatively long period of time for the piston to be braked and tends to lower the accuracy with which the piston is stopped by the brake mechanism. Another difficulty is that a piping system connected between the cylinder, the brake mechanism, and their solenoid-operated directional control valves is complex.
In the case where the piston of the actuator system is directed vertically, the workpiece conveyed by the actuator is likely to drop abruptly in the event of an electric power failure or a fluid pressure supply failure.
In the actuator system, the electric motor is directly coupled to the ball screw. The piston rod connected to the piston rod, the ball screw, and the drive shaft of the electric motor are thus connected in series with each other. Such a series-connected shaft arrangement is disadvantageous in that the actuator system has too a long of an axial dimension under some installing conditions.